Digital rock analysis systems and methods with reliable multiphase permeability determination

Abstract

The pore structure of rocks and other materials can be determined through microscopy and subjected to digital simulation to determine the properties of multiphase fluid flows through the material. To ensure reliable results, the digital rock model is first analyzed via a series of operations that, in some embodiments, include: obtaining a three-dimensional pore/matrix model of a sample; determining a flow axis; verifying that the dimension of the model along the flow axis exceeds that of a representative elementary volume (REV); selecting a flow direction; extending model by mirroring if pore statistics at a given saturation are mismatched for different percolating phases; and increasing resolution if the smallest nonpercolating sphere dimension is below a predetermined threshold. This sequence of operations increases reliability of results when measuring relative permeability using the model and displaying relative permeability measurements to user.

What is claimed is:  
     
       1. A multiphase permeability determination method performed by one or more computers, the method comprising:
 obtaining a three-dimensional pore and matrix model of a rock formation sample based on digital imaging of the sample using a scanning microscope; 
 determining an axis of flow passing through said sample, wherein the determination is based on the orientation of the sample, formation pressure gradiens, or user specifications; 
 verifying that the dimension of the model along said axis exceeds that of a representative elementary volume (REV); 
 selecting a direction of the flow; 
 extending said model by mirroring if pore statistics at a given saturation are mismatched for different percolating phases; 
 increasing a resolution of digital imaging of the sample and repeating said obtaining, determining, verifying, selecting, extending, and increasing, if a smallest non-percolating sphere dimension is below a predetermined threshold; 
 measuring relative permeability of said sample using the model; and 
 providing the relative permeability measurements to a user interface for display to a user. 
 
     
     
       2. The method of  claim 1 , further comprising: determining absolute permeability along each axis before said determining the axis of flow. 
     
     
       3. The method of  claim 1 , wherein said selecting a direction of the flow includes:
 examining opposing faces perpendicular to the axis of flow to eliminate any face having large pores near a boundary; and 
 if two faces remain, selecting the face having better pore homogeniety as a preferred inlet face. 
 
     
     
       4. The method of  claim 3 , further comprising:
 determining a mercury injection capillary pressure (MICP) saturation at which connectivity occurs for each direction along said axis of flow; and 
 verifying that a strong directional dependence does not exist. 
 
     
     
       5. The method of  claim 1 , wherein said selecting a direction of the flow includes:
 determining a mercury injection capillary pressure (MICP) saturation at which connectivity occurs for each direction along said axis of flow; and 
 verifying that a strong directional dependence does not exist. 
 
     
     
       6. The method of  claim 1 , wherein the predetermined threshold is a diameter of 3 voxels. 
     
     
       7. The method of  claim 1 , wherein said obtaining includes:
 scanning a physical rock sample using the scanning microscope to obtain a three-dimensional digital image; and 
 deriving the pore and matrix model from the three-dimensional image. 
 
     
     
       8. The method of  claim 1 , wherein said pore statistics include a distribution of standard deviation of porosity. 
     
     
       9. The method of  claim 8 , wherein said pore statistics further include a distribution of standard deviation of pore surface to volume ratio. 
     
     
       10. A multiphase permeability determination system that comprises:
 a user interface; 
 a memory having software; and 
 one or more processors coupled to the memory to execute the software, the software causing the one or more processors to:
 obtain a three-dimensional pore and matrix model of a rock formation sample based on digital imaging of the sample using a scanning microscope; 
 determine an axis of flow passing through said sample, wherein the determination is based on the orientation of the sample, formation pressure gradiens, or user specifications; 
 verify that the dimension of the model along said axis exceeds that of a representative elementary volume (REV); 
 select a direction of the flow; 
 extend said model by mirroring if pore statistics at a given saturation are mismatched for different percolating phases; 
 increase the resolution of digital imaging of the sample and repeat said obtain, determine, verify, select, extend, and increase steps if a smallest non-percolating sphere dimension is below a predetermined threshold; 
 measure relative permeability of said sample using the model; and 
 provide the relative permeability measurements to the user interface for display to a user. 
 
 
     
     
       11. The system of  claim 10 , wherein the software further causes the one or more processors to determine absolute permeability along each axis before said determining the axis of flow. 
     
     
       12. The system of  claim 10 , wherein as part of selecting a direction of the flow, the software causes the one or more processors to:
 examine opposing faces perpendicular to the axis of flow to eliminate any face having large pores near a boundary; and 
 if two faces remain, select the face having better pore homogeniety as a preferred inlet face. 
 
     
     
       13. The system of  claim 12 , wherein the software further causes the one or more processors to:
 determine a mercury injection capillary pressure (MICP) saturation at which connectivity occurs for each direction along said axis of flow; and 
 verify that a strong directional dependence does not exist. 
 
     
     
       14. The system of  claim 10 , wherein said as part of selecting a direction of the flow, the software causes the one or more processors to:
 determine a mercury injection capillary pressure (MICP) saturation at which connectivity occurs for each direction along said axis of flow; and 
 verify that a strong directional dependence does not exist. 
 
     
     
       15. The system of  claim 10 , wherein the predetermined threshold is a diameter of 3 voxels. 
     
     
       16. The system of  claim 10 , wherein as part of obtaining a three-dimensional pore and matrix model, the software causes the one or more processors to:
 retrieve a three-dimensional image of a physical rock sample generated using the scanning microscope; and 
 derive the pore and matrix model from the three-dimensional image. 
 
     
     
       17. The system of  claim 10 , wherein said pore statistics include a distribution of standard deviation of porosity. 
     
     
       18. The system of  claim 17 , wherein said pore statistics further include a distribution of standard deviation of pore surface to volume ratio.